[TypeScript] Error "Namespace not found for return type" for NOTION.COM api

### What are you generating using Kiota, clients or plugins?

API Client/SDK

### In what context or format are you using Kiota?

Nuget tool

### Client library/SDK language

None

### Describe the bug

Found as part of #7774

It fails with an error:

```
crit: Kiota.Builder.KiotaBuilder[0]
error generating the client: Namespace not found for return type
System.InvalidOperationException: Namespace not found for return type
at Kiota.Builder.Writers.TypeScript.CodeFunctionWriter.FindFunctionInNameSpace(String functionName, CodeElement codeElement, CodeType returnType) in E:\Projekte\Github\kiota\src\Kiota.Builder\Writers\TypeScript\CodeFunctionWriter.cs:line 443
at Kiota.Builder.Writers.TypeScript.CodeFunctionWriter.GetDeserializerFunctionName(CodeElement codeElement, CodeType returnType) in E:\Projekte\Github\kiota\src\Kiota.Builder\Writers\TypeScript\CodeFunctionWriter.cs:line 436
at Kiota.Builder.Writers.TypeScript.CodeFunctionWriter.WriteComposedTypeDeserializer(CodeFunction codeElement, LanguageWriter writer, CodeParameter composedParam, CodeFile codeFile) in E:\Projekte\Github\kiota\src\Kiota.Builder\Writers\TypeScript\CodeFunctionWriter.cs:line 154
at Kiota.Builder.Writers.TypeScript.CodeFunctionWriter.WriteDeserializerFunction(CodeFunction codeFunction, CodeFile codeFile, LanguageWriter writer) in E:\Projekte\Github\kiota\src\Kiota.Builder\Writers\TypeScript\CodeFunctionWriter.cs:line 667
at Kiota.Builder.Writers.TypeScript.CodeFunctionWriter.WriteCodeElement(CodeFunction codeElement, LanguageWriter writer) in E:\Projekte\Github\kiota\src\Kiota.Builder\Writers\TypeScript\CodeFunctionWriter.cs:line 115
at Kiota.Builder.Writers.LanguageWriter.Write[T](T code) in E:\Projekte\Github\kiota\src\Kiota.Builder\Writers\LanguageWriter.cs:line 149
at Kiota.Builder.CodeRenderers.CodeRenderer.RenderCode(LanguageWriter writer, CodeElement element) in E:\Projekte\Github\kiota\src\Kiota.Builder\CodeRenderers\CodeRenderer.cs:line 83
at Kiota.Builder.CodeRenderers.CodeRenderer.RenderCode(LanguageWriter writer, CodeElement element) in E:\Projekte\Github\kiota\src\Kiota.Builder\CodeRenderers\CodeRenderer.cs:line 88
at Kiota.Builder.CodeRenderers.CodeRenderer.RenderCodeNamespaceToSingleFileAsync(LanguageWriter writer, CodeElement codeElement, String outputFile, CancellationToken cancellationToken) in E:\Projekte\Github\kiota\src\Kiota.Builder\CodeRenderers\CodeRenderer.cs:line 35
at Kiota.Builder.CodeRenderers.CodeRenderer.RenderCodeNamespaceToSingleFileAsync(LanguageWriter writer, CodeElement codeElement, String outputFile, CancellationToken cancellationToken) in E:\Projekte\Github\kiota\src\Kiota.Builder\CodeRenderers\CodeRenderer.cs:line 37
at Kiota.Builder.CodeRenderers.CodeRenderer.RenderCodeNamespaceToFilePerClassAsync(LanguageWriter writer, CodeNamespace currentNamespace, CancellationToken cancellationToken) in E:\Projekte\Github\kiota\src\Kiota.Builder\CodeRenderers\CodeRenderer.cs:line 55
at Kiota.Builder.CodeRenderers.CodeRenderer.RenderCodeNamespaceToFilePerClassAsync(LanguageWriter writer, CodeNamespace currentNamespace, CancellationToken cancellationToken) in E:\Projekte\Github\kiota\src\Kiota.Builder\CodeRenderers\CodeRenderer.cs:line 59
at Kiota.Builder.CodeRenderers.CodeRenderer.RenderCodeNamespaceToFilePerClassAsync(LanguageWriter writer, CodeNamespace currentNamespace, CancellationToken cancellationToken) in E:\Projekte\Github\kiota\src\Kiota.Builder\CodeRenderers\CodeRenderer.cs:line 59
at Kiota.Builder.CodeRenderers.CodeRenderer.RenderCodeNamespaceToFilePerClassAsync(LanguageWriter writer, CodeNamespace currentNamespace, CancellationToken cancellationToken) in E:\Projekte\Github\kiota\src\Kiota.Builder\CodeRenderers\CodeRenderer.cs:line 59
at Kiota.Builder.KiotaBuilder.CreateLanguageSourceFilesAsync(GenerationLanguage language, CodeNamespace generatedCode, CancellationToken cancellationToken) in E:\Projekte\Github\kiota\src\Kiota.Builder\KiotaBuilder.cs:line 668
at Kiota.Builder.KiotaBuilder.<>c__DisplayClass21_0.<b__0>d.MoveNext() in E:\Projekte\Github\kiota\src\Kiota.Builder\KiotaBuilder.cs:line 311
--- End of stack trace from previous location ---
at Kiota.Builder.KiotaBuilder.GenerateConsumerAsync(Func`5 innerGenerationSteps, CancellationToken cancellationToken) in E:\Projekte\Github\kiota\src\Kiota.Builder\KiotaBuilder.cs:line 348
at Kiota.Builder.KiotaBuilder.GenerateConsumerAsync(Func`5 innerGenerationSteps, CancellationToken cancellationToken) in E:\Projekte\Github\kiota\src\Kiota.Builder\KiotaBuilder.cs:line 368
at Kiota.Builder.KiotaBuilder.GenerateClientAsync(CancellationToken cancellationToken) in E:\Projekte\Github\kiota\src\Kiota.Builder\KiotaBuilder.cs:line 285
at kiota.Handlers.KiotaGenerateCommandHandler.InvokeAsync(ParseResult parseResult, CancellationToken cancellationToken) in E:\Projekte\Github\kiota\src\kiota\Handlers\KiotaGenerateCommandHandler.cs:line 166
```

### Expected behavior

No error, client should be generated

### How to reproduce

After having built Kiota, execute this:

```
.\it\generate-code.ps1 -descriptionUrl https://developers.notion.com/openapi.json -language "typescript" -dev
```

### Open API description file

https://developers.notion.com/openapi.json

### Kiota Version

source code build dated 2026-06-15

### Latest Kiota version known to work for scenario above?(Not required)

_No response_

### Known Workarounds

_No response_

### Configuration

_No response_

### Debug output

_No response_

### Other information

_No response_

The failure points to src/Kiota.Builder/Writers/TypeScript/CodeFunctionWriter.cs, especially FindFunctionInNameSpace and GetDeserializerFunctionName. Reproduce it with it/generate-code.ps1 and the Notion OpenAPI description; done means the TypeScript client generates without the namespace error.

Written by the indexing model from the issue text.